1. Technical Field
The present invention relates generally to an apparatus for measuring the magnetic properties of magnetic materials and particularly concerns an apparatus that is capable of highly sensitive and non-destructive testing of the M-H hysteresis loop on a wide variety of magnetic materials.
2. Discussion
Currently available techniques for measuring the M-H hysteresis loop properties of magnetic materials require test samples to be produced to a specific geometry. An instrument such as a vibrating sample magnetometer (VSM), loop tracer, or hysteresigraph is then used to measure the M-H hysteresis loop of the sample. A disadvantage associated with these techniques is the inconvenience of sample preparation given the many different forms of magnetic products to be tested. Many commercial magnetic products have the form of thin sheets such as magnetic recording tape, hard disks for computer data storage and laminations used for transformers. Producing a sample of specific size suitable for testing from these products, using a cutting, punching or forming process, may stress the material, thereby changing the magnetic properties being measured due to magnetostriction. In addition, these methods of sample preparation may cause ferromagnetic contamination around the edges or on other surfaces, which can cause significant error in measurement of the hysteresis loop, especially in small or weakly-magnetic samples. The preparation of samples and measurement of M-H loop hysteresis properties with a vibrating sample magnetometer (VSM) is also time-consuming, usually requiring 20 to 40 minutes per sample. Test methods which destroy the samples so that they can no longer be used are often impractical or unusable in applications where process control testing is desired.
Other difficult and time-consuming sample preparations are sometimes required, which make prior testing techniques impractical. For example, the testing of amorphous ribbon laminations requires that a sample strip be rolled in the shape of a toroid and that primary and secondary windings be wound on the toroid. Sample preparation is time-consuming and can distort the magnetic properties of the material.
Several attempts have been made in the prior art to improve the accuracy and speed of M-H hysteresis loop testers. LDJ Electronics, Inc. of Michigan, sells an M-H hysteresis loop tester that measures the magnetic properties of computer data storage disks non-destructively. This device tests a large portion of the disk by applying a magnetic field to the entire disk. It then measures the resulting magnetization change across the diameter of the disk. This technique has the disadvantage of measuring both sides of the disk at the same time, as it is not capable of measuring the M-H hysteresis loop over a small section of the disk. Further, this technique is limited to measuring products which fit into the drive coil of the testing device.
Magneto-Optic hysteresis loop testers measure the magnetic characteristics of the surface of a magnetic film by the Kerr effect. A disadvantage to the Magneto-Optic technique, however, is that it cannot detect the additional magnetization flux below the reflective surface of the magnetic film. Magnetization-times-thickness product is a figure of merit relating to available magnetic signal strength and is an important parameter for controlling the manufacturing process. Since a Magneto-Optic system does not respond to film thickness, it cannot measure this important parameter.
Another device sold by Innovative Instrumentation, Inc. uses a technique that mimics magnetic recording where a signal is written to a computer data storage disk. The disk is rotated past a sensor that records the magnetic intensity of the signal. With each rotation, the write signal is increased and the sensor measures the resultant induction signal. This technique, however, is slow and cumbersome and cannot measure the actual M-H hysteresis loop of the material.
An alternative prior art testing device is disclosed in U.S. Pat. No. 4,843,316 to Hesterman. The Hesterman apparatus is a non-destructive M-H hysteresis tester for magnetic computer disks which utilizes a series of balanced drive coils about a magnetic core to magnetize a sample, which in turn produces a voltage signal detected by a sense coil located in the center of the magnetic core. This apparatus is easier and less time-consuming for testing magnetic properties because of the elimination of sample preparation, and is also useful for applications where destruction of the material cannot be allowed. However, the shortcomings associated with this device involve its inability to produce sufficient sensitivity while overcoming background electrical and magnetic noise for measuring thin film samples such as those of less than one micrometer thickness, or other samples that are weakly magnetic and therefore produce low signal to noise ratios. In fact, all currently available magnetic testing systems lack the ability to increase the signal to noise ratio to produce sufficient sensitivity for testing thin or weakly magnetic samples.
The need therefore exists for an improved method and apparatus for non-destructively testing magnetic samples with increased sensitivity.